A major problem in formulating biologically active compounds is their poor solubility or insolubility in water. For instance, over one third of the drugs listed in the United States Pharmacopoeia are either water-insoluble or poorly water-soluble. Oral formulations of water-insoluble drugs or compounds with biological uses frequently show poor and erratic bioavailability. In addition, drug insolubility is one of the most recalcitrant problems facing medicinal chemists and pharmaceutical scientists developing new drugs. Water-insolubility problems delay or completely block the development of many new drugs and other biologically useful compounds, or prevent the much-needed reformulation of certain currently marketed drugs. Although the water-insoluble compounds may be formulated by solubilization in organic solvents or aqueous-surfactant solutions, in many cases such solubilization may not be a preferred method of delivery of the water-insoluble agent for their intended biological use. For instance, many currently available injectable formulations of water-insoluble drugs carry important adverse warnings on their labels that originate from detergents and other agents used for their solubilization.
An alternative approach for the formulation of water-insoluble biologically active compounds is surface-stabilized particulate preparations. Small particle size formulation of drugs are often needed in order to maximize surface area, bioavailability and, dissolution requirements. Pace et al. ("Novel Injectable Formulations of Insoluble Drugs" in Pharmaceutical Technology, March 1999) have reviewed the usefulness of the microparticulate preparations of water-insoluble or poorly soluble injectable drugs.
In U.S. Pat. Nos. 5,091,187 and 5,091,188 to Haynes describe the use of phospholipids as surface stabilizers to produce aqueous suspension of submicron sized particles of the water-insoluble drugs. These suspensions are believed to be the first applications of the surface modified microparticulate aqueous suspension containing particles made up of a core of pure drug substances and stabilized with natural or synthetic bipolar lipids including phospholipids and cholesterol. Subsequently, similar delivery systems exploiting these principles have been described (G. G. Liversidge et al., U.S. Pat. No. 5,145,684; K. J. Illig et al. U.S. Pat. No. 5,340,564 and H. William Bosch et al., U.S. Pat. No. 5,510,118) emphasizing the usefulness of the drug delivery approach utilizing particulate aqueous suspensions.
In U.S. Pat. No. 5,246,707 Haynes teaches uses of phospholipid-coated microcrystals in the delivery of water-soluble biomolecules such as polypeptides and proteins. The proteins are rendered insoluble by complexation and the resulting material forms the solid core of the phospholipid-coated particle.
These patents and others utilized processes based on the particle size reduction by mechanical means such as attrition, cavitation, high-shear, impaction, etc achieved by media milling, high pressure homogenization, ultrasonication, and microfluidization of aqueous suspensions. However, these particle size reduction methods suffer from certain disadvantage, such as long process duration (high-pressure homogenization or microfluidization) and contamination (media milling, and ultrasonication). In addition, these methods may not be suitable for aqueous suspensions of compounds with limited stability in aqueous medium at the pH, high temperature and high pressure conditions prevailing in these processes.
Among the alternatives that address to these problems is a procedure which uses liquefied gasses for the production of microparticulate preparations. In one such method liquefied-gas solutions are sprayed to form aerosols from which fine solid particles precipitate. The phenomenon of solids precipitated from supercritical fluids was observed and documented as early as 1879 by Hannay, J. B. and Hogarth, J. "On the Solubility of Solids in Gases," Proc. Roy. Soc. London 1879 A29, 324,
The first comprehensive study of rapid expansion from a liquefied-gas solution in the supercritical region was reported by Krukonis (1984) who formed micro-particles of an array of organic, inorganic, and biological materials. Most particle sizes reported for organic materials, such as lovastatin, polyhydroxyacids, and mevinolin, were in the 5-100 micron range. Nanoparticles of beta-carotene (300 nm) were formed by expansion of ethane into a viscous gelatin solution in order to inhibit post expansion particle aggregation. Mohamed, R. S., et al. (1988), "Solids Formation After the Expansion of Supercritical Mixtures," in Supercritical Fluid Science and Technology, Johnston, K. P. and Penninger, J. M. L., eds., describes the solution of the solids naphthalene and lovastatin in supercritical carbon dioxide and sudden reduction of pressure to achieve fine particles of the solute. The sudden reduction in pressure reduces the solvent power of the supercritical fluid, causing precipitation of the solute as fine particles.
Tom, J. W. and Debenedetti, P. B. (1991), "Particle Formation with Supercritical Fluids--a Review," J. Aerosol. Sci. 22:555-584, discusses rapid expansion of supercritical solutions techniques and their applications to inorganic, organic, pharmaceutical and polymeric materials. This technique is useful to comminute shock-sensitive solids, to produce intimate mixtures of amorphous materials, to form polymeric microspheres and deposit thin films.
Most studies of rapid expansion from supercritical solution on organic materials utilize supercritical carbon dioxide. However, ethane was preferred to carbon dioxide for beta-carotene because of certain chemical interactions. Carbon dioxide is generally preferred, alone or in combination with a cosolvent. Minute additions of a cosolvent can significantly influence the solvent properties. When cosolvents are used in rapid expansion from a supercritical solution, care is required to prevent de-solution of the particles due to solvent condensing in the nozzle. Normally, this is achieved by heating the supercritical fluid, prior to expansion, to a point where no condensate (mist) is visible at the nozzle tip.
A similar problem occurs when carbon dioxide is used. During adiabatic expansion (cooling), carbon dioxide will be in two phases unless sufficient heat is provided at the nozzle to maintain a gaseous state. Most investigators recognize this phenomenon and increase the pre-expansion temperature to prevent condensation and freezing in the nozzle. A significant heat input is required (40-50 kcal/kg) to maintain carbon dioxide in the gaseous state. If this energy is supplied by increasing the pre-expansion temperature the density drops and consequently reduces the supercritical fluid's solvating power. This can lead to premature precipitation and clogging of the nozzle.
The solvent properties of liquefied-gas are strongly affected by their fluid density in the vicinity of the fluid's critical point. In rapid expansion from liquefied-gas solutions, a non-volatile solute is dissolved in a liquefied-gas that remains either in the supercritical or sub-critical phase. Nucleation and crystallization are triggered by reducing the solution density through rapid expansion of the liquefied-gas to atmospheric conditions. To achieve this the liquefied-gas is typically sprayed through 10-50 micron (internal diameter) nozzles with aspect ratios (L/D) of 5-100. High levels of supersaturation result in rapid nucleation rates and limited crystal growth. The combination of a rapidly propagating mechanical perturbation and high supersaturation is a distinguishing feature of rapid expansion from a liquefied-gas solution. These conditions lead to the formation of very small particles with a narrow particle size distribution.
There are a number of advantages in utilizing compressed carbon dioxide in the liquid and supercritical fluid states, as a solvent or anti-solvent for the formation of materials with submicron particle features. Diffusion coefficients of organic solvents in supercritical fluid carbon dioxide are typically 1-2 orders of magnitude higher than in conventional liquid solvents. Furthermore, carbon dioxide is a small linear molecule that diffuses more rapidly in liquids than do other antisolvents. In the antisolvent precipitation process, the accelerated mass transfer in both directions can facilitate very rapid phase separation and hence the production of materials with sub-micron features. It is easy to recycle the supercritical fluid solvent at the end of the process by simply reducing pressure. Since supercritical fluids do not have a surface tension, they can be removed without collapse of structure due to capillary forces. Solvent removal from the product is unusually rapid. No carbon dioxide residue is left in the product, and carbon dioxide has a number of other desirable characteristics, for example it is non-toxic, nonflammable, and inexpensive. Furthermore, solvent waste is greatly reduced since a typical ratio of antisolvent to solvent is 30:1.
Exploiting these concepts Henriksen et al. in WO 97/14407, disclosed a process using compressed fluids to produce sub-micron sized particles of water insoluble compounds with biological uses, particularly water insoluble drugs by precipitating a compound by rapid expansion from a supercritical solution in which the compound is dissolved, or precipitating a compound by spraying a solution, in which the compound is soluble, into compressed gas, liquid or supercritical fluid which is miscible with the solution but is antisolvent for the compound. In this manner precipitation with a compressed fluid antisolvent (compressed fluid antisolvent) is achieved.
An essential element of this process is the use of phospholipids and other surface modifiers to alter the surface of the drug particles to prevent particle aggregation and thereby improve both their storage stability and pharmacokinetic properties. This process combines or integrates phospholipids or other suitable surface modifiers such as surfactants, as the aqueous solution or dispersion in which the supercritical solution is sprayed. The surfactant is chosen to be active at the compound-water interface, but is not chosen to be active at the carbon dioxide-organic solvent or carbon dioxide-compound interface when carbon dioxide is used as the supercritical solution. The use of surface modifying agents in the aqueous medium allowed making submicron particles by the compressed fluid antisolvent process without particle aggregation or flocculation.